An atom probe, also referred to as an atom probe microscope, is a device which allows specimens to be analyzed on an atomic level. A typical atom probe includes a vacuum chamber containing a specimen mount, a detector opposite the specimen mount, and a counter electrode between the specimen mount and the detector. (The counter electrode is sometimes referred to as a “local electrode” or “extraction electrode”; additionally, because electrodes in an atom probe typically serve as electrostatic lenses, the term “lens” is sometimes used in place of the term “electrode.”) During typical analysis, a specimen is situated in the specimen mount when the chamber is under high vacuum, with the specimen being cryogenically cooled to better “settle” its atoms and minimize their thermal motion. A positive electrical charge (e.g., a baseline voltage) is applied to the specimen such that the electrostatic field near the apex of the specimen—the portion of the specimen facing the detector—is near that required to spontaneously ionize atoms on the surface of the apex. The detector and counter electrode are each either grounded or negatively charged. A positive electrical pulse (above the baseline voltage), a laser pulse (e.g., photonic energy), and/or another pulsed form of ionization energy (e.g., an electron beam or packet, ion beam, RF pulse, etc.) is intermittently applied to the specimen to increase the probability that surface atoms on the specimen will ionize. Alternatively or additionally, a negative voltage pulse can be applied to the counter electrode in synchrony with the foregoing energy pulse(s). Occasionally, a pulse will cause ionization of a single atom from the apex of the specimen. The ionized atom(s) separate or “evaporate” from the apex, pass through an aperture in the counter electrode, and impact the surface of the detector, typically a microchannel plate (MCP). The elemental identity of an ionized atom can be determined by measuring its time of flight (TOF), that is, the time between the pulse that liberates the ion from the specimen apex and the time it impinges on the detector. The velocity of an ionized atom (and thus its TOF) varies based on the mass-to-charge-state ratio (m/n) of the atom/ion, with lighter and/or more highly charged ions taking less time to reach the detector. Since the TOF of an ion is indicative of the mass-to-charge ratio of the ion, which is in turn indicative of elemental identity, the TOF can help identify the composition of the ionized atom. In addition, the location of the ionized atom on the surface of the specimen can be determined by measuring the location of the atom's impact on the detector. Thus, as the specimen is evaporated, a three-dimensional map or image of the specimen's constituent atoms can be constructed. While the image represented by the map is a point projection, with atomic resolution and a magnification of over 1 million times, the map/image data can be analyzed in virtually any orientation, and thus the image can be considered tomographic in nature. Further details on atom probes can be found, for example, in U.S. Pat. Nos. 5,440,124; 7,157,702; 7,683,318; 7,884,323; 8,074,292; 8,276,210; 8,513,597; and 8,575,544, as well as in the patents and other literature noted on the first page of this document (and the patents and other literature referenced therein).
High vacuum is needed in the vacuum chamber because gases, water, and other contaminants present in the vacuum chamber can significantly degrade image quality. Such contaminants can impinge on the detector and mistakenly be interpreted as ions, and can interfere with ion flight. Even with high vacuum, “tramp gas”—that is, the small amount of residual contaminants left in the vacuum chamber after evacuation—creates noise in the image data, primarily from adsorption of such contaminants on the cryogenically-cooled specimen, with subsequent ionization of these contaminants then being detected as if they were components of the specimen. The vacuum chamber is therefore typically evacuated to ultra-high vacuum (UHV) conditions (chamber pressure below about 10−8 Pa), first by using conventional vacuum pumps to achieve medium/high vacuum (chamber pressure below about 10−3 Pa), and then by utilizing high vacuum pumps such as ion pumps (which ionize and electrically collect gases), cryopumps (which condense gases onto cold surfaces), getters and getter/sublimation pumps (which use materials which bond to or adsorb gases), and/or turbomolecular pumps (which mechanically propel gases from the vacuum chamber). The higher the vacuum—that is, the fewer stray molecules/atoms present in the vacuum chamber—the better the imaging results. However, achieving high vacuum is time-consuming, and has a significant impact on atom probe operating expense.